Most modern integrated circuit microcomputers contain a random access memory designed to contain temporary data or instructions for operation of the microcomputer. The data stored in a static random access memory is destroyed or lost whenever the current through the memory drops below a minimum data sustaining level, and an excessive current through the memory load transistor affects the reliability of the memory by limiting the useful life of the transistor by overheating. A proper load current is therefore necessary.
The load current in a typical N-channel silicon gate MOS transistor circuit is determined by the threshold voltage of the depletion load device. A greater negative bias voltage increases the threshold voltage and results in lower load currents. Conversely, a more positive bias voltage decreases the threshold voltage and results in higher load current. For proper regulation of the memory load current, it is necessary to provide regulation of the bias voltage generating circuits.
The circuitry disclosed herein provides and maintains proper bias voltage regulation for an associated static random access memory to therefore provide a proper data sustaining current through the memory.